The use of charged particles as filtering aids in the membrane filtration of liquid cell culture media such as fermentation liquors is known. U.S. Pat. No. 4,200,659 to Chong et al describes flocs prepared by mixing microsized, positively and negatively charged particles and the use of these preformed flocs as filtering agents for fermentation broths, and suggests that negatively or positively charged particles individually may be ultrafiltered within the lumens of fine hollow fibers (col. 9, line 59 to col. 10, line 44). In another context (col. 12, lines 17-30), the Chong et al. patent describes preparation of the flocs in the liquid to be treated and filtered by adding negatively and positively charged particles to the liquid.
Combinations of fine-particle-size, positively and negatively charged polymeric adsorbents have also been used sequentially for other purposes, such as the removal of humic substances from potable water which react with chlorine to produce toxic trihalomethanes, as described in U.S. Pat. No. 4,537,683 to Isacoff and Neely, or simultaneous decolorization and clarification of impure sugar solutions, as described by Cartier, U.S. Pat. No. 4,247,340.